1. Field of the Invention
The present invention relates to a drive element for the platen of a typewriter or similar office machine having a movable carriage, the drive element being in operative contact via a ratchet with a line space wheel disposed on the shaft of the platen and being acted upon by a force causing the movement of the platen based on the movements of the carriage.
2. The Prior Art
In general, two different systems for turning the platen by means of drives disposed on the machine are customary in machines of the above mentioned type. One system provides a motor in the machine which acts via a corresponding drive element on a toothed wheel which is connected, fixed against rotation, with the platen. The advantage of this system lies in general in that, given a suitable motor and a suitable control, the platen can be turned around angles of rotation of almost any desired smallness. This solution is disadvantageous in that high costs are involved.
The second system for performing the paper transport by turning of the platen provides a line space wheel disposed on the platen and connected with it, fixed against rotation, which is in an operative contact with a ratchet. Often a magnet disposed on the machine is provided for the operation of the ratchet which pulls the ratchet, which is acted upon by a spring, out of its position of rest into its operative position and thus turns the line space wheel and, along with it, the platen by a set angle of rotation. Another solution provides that the ratchet, which is also acted upon by a spring, is connected via a drive chain with a pivot element, which is pivotable by means of the movement of the carriage and pulls in this way the ratchet out of its position of rest into its operative position, so that here, too, the platen is turned by a set angle of rotation. Such an arrangement can be found, for example, in German Published, Nonexamined Application DE-OS 20 59 154.
Compared with the others, the last recited possibility of turning the platen and thus accomplishing the movement of the paper is by far the most cost effective. However, it is disadvantageous that a very great effort is required to provide bearings for the individual drive elements and furthermore, that inexactness because of wear must be expected.